89 GMC S15 V6 going down the road and motor dies

Can start up truck in the mornings and will run fine for about 20 minutes then die while going down the street. I can let it sit for a few minutes and it will start up and run for about a minute and die again.I have replaced the coil and the fuel pump but the problem is still there.I pulled the distributor wire and checked to see if it was firing while turning over, sometimes it will fire and sometimes it will not.

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There is a black fuel relay way up under the top left driver side dash that breaks its main wire solder connections on the circuit board under over-heat conditions. Resolder that relay. Starting fluid will show that you have lost fuel delivery under no start conditions. You can use the breather hose from the valve cover for this test.

F150 is a Ford model truck, not GMC.
But at any rate, since I don't know if you have a Ford F150 of some kind, a 87 GMC S15 or some kind of 89 GMC with a V6 I will suggest taking the ignition module to an Auto Zone and having them test it (A free service). Also test the fuel pressure.
If a tune up and fuel filter has not been done in some time that could very well be an issue.

This is most likely an ignition module located inside the distributor that is failing intermittently, this truck should be brought to a Auto Zone to check for trouble codes that are stored, they do the code check for free and this will help narrow down the problem so you won't be changing parts that are not bad.